I made and uploaded a blood red bath water replacement to MTS2 a while ago after a request. Pescado did indicate that he was interested in making a stand-alone version, but since he still hasn't I had to do one myself.
I originally tried it with one of the cheaper tubs, but the maxis bath water showed through my red texture, and I couldn't track it down and stomp it out. However, this more expensive tub works fine as a stand-alone object.
Let me know if any weirdness occurs. It shouldn't, since it's a simple cloned object.
I have not messed with any BHAVs, but anyone who wants to give it a go is fee to mess with this in any way they want.